Chemistry of Life unit test
A test on this unit alone, marked as a percentage and a letter grade — for the test your class is actually sitting, rather than for May. Answer everything, then submit once: seeing the answer to question 3 before attempting question 4 makes the final percentage meaningless.
Nucleotide structure
Buffer
Lipid structure
Carbohydrate structure and function
Denaturation
Nucleic acid directionality
Dehydration synthesis
Antiparallel strands
Peptide bond
Saturated vs unsaturated fat
Disulfide bridge
Directionality and complementarity
Short answer 1. Define or explain: Phospholipid amphipathicity
3 ptsShort answer 2. Define or explain: Carbonic acid–bicarbonate buffer
3 ptsShort answer 3. Define or explain: Surface tension
3 ptsShort answer 4. Define or explain: Monomers of the four macromolecules
3 ptsFree response
9 ptsMolecules known as dinucleoside polyphosphates, a type of nucleotide, are signaling molecules that accumulate in plant cells during dry or stressful conditions. Stomata are pores on the surface of leaves that open in light and regulate the flow of carbon dioxide, oxygen and water vapor into and out of a plant. In dry conditions many plants close their stomata, reducing water loss. Scientists hypothesized that the dinucleoside polyphosphates Ap4A and Cp4C bind to DORN1 receptors on the plasma membrane of plant cells and initiate signaling cascades that close stomata. They exposed plants to light to open the stomata, then placed leaf samples in buffer alone or in buffer containing Ap4A, Cp4C, or abscisic acid (ABA) — a molecule known to close stomata. Stomatal size was measured relative to the buffer-alone sample. FIGURE 1. RELATIVE SIZE OF STOMATA, width/length (± SE of the mean) Treatment Relative size of stomata Buffer 1.00 ± 0.05 Ap4A 0.65 ± 0.05 Cp4C 0.63 ± 0.05 ABA 0.70 ± 0.06 In a second experiment the scientists used plants identical to those above except that the gene encoding the DORN1 receptor was mutated, and repeated the procedure. FIGURE 2. RELATIVE SIZE OF STOMATA IN DORN1 MUTANTS (± SE of the mean) Treatment Relative size of stomata Buffer 1.00 ± 0.05 Ap4A 1.10 ± 0.10 Cp4C 0.70 ± 0.05 ABA 0.60 ± 0.05
(a) Describe the three structural components of a nucleotide.
(b)(i) Identify the dependent variable in the scientists’ first experiment.
(b)(ii) Based on Figure 1, describe the relative size of the stomata in the Cp4C treatment group as compared with the size of the stomata in the group treated with buffer alone.
(b)(iii) Justify the use of buffer alone as a control in the scientists’ experiments.
(c)(i) Justify the scientists’ treating one sample of leaves with ABA in the experiments.
(c)(ii) Based on Figure 2, describe the difference between the effects of Ap4A and Cp4C treatments on the size of stomata in plants with mutated DORN1 receptors.
(c)(iii) Activation of DORN1 receptors induces plant cells to produce certain molecules that cause the stomata to close. Based on the data in Figures 1 and 2 for cells treated with Ap4A, predict the relative production of those molecules by DORN1-mutated cells compared with production by nonmutated cells.
(d)(i) The DORN1 receptor transmits signals that increase transcription of genes involved in plant defenses. The scientists developed DORN1-mutant plants in which the receptor lacked most of its intracellular domain. In cells homozygous for this mutation, predict the effect of Ap4A treatment on transcription of the plant-defense genes relative to the effect of Ap4A on nonmutated cells.
(d)(ii) Justify your prediction in (d)(i).